As a part of the Magistracy of Canopus task force, the 58th was stationed on Tellman's Mistake alongside the other elements of the 20th Division when military action against the Magistracy began. The Commanding Officer of VII Corps, Captain-GeneralMarion Marik, would often assign a division or more of troops to the conquest of various worlds by drawing together brigades from across VII Corps. The 58th often operated in conjunction with other brigades from VII Corps as well as fighting alongside units from the Free Worlds League Military assigned to the task force as auxiliary forces.[1][2]

The 58th was assigned to the conquest of Cranston in one of the first campaigns of the war. Cranston was expected to be a tough battle, and the 58th had already been ordered to redeploy most of the brigade's fighters to the 57th Brigade for their assault on the nearby world of Restitution. To bolster the 58th, the 2nd Orloff Grenadiers and the 1st Marik Militia were deployed alongside them from the auxiliary FWLM contingent. Defending Cranston when the 58th landed in October 2577 were the 1st Canopian Grenadiers, who the 58th soon discovered were much better armed and equipped than SLDF intelligence had indicated. Although the 58th and the FWLM units had the Canopian Grenadiers heavily outnumbered, the Canopians played to their own strengths by using maneuverability, speed and the local terrain to constantly harass the SLDF units, hitting hard and fading back before the 58th or the Marik troops could bring their superior numbers to bear.[3][4]

Hampered by a lack of AeroSpace forces to do any reconnaissance or gather intelligence and facing a foe that refused to engage in stand-up battles dragged the 58th's campaign out into November; events then took a turn for the worse when, on the 9th of November, the SLDF WarShip picket line in the system reported an unidentified vessel arriving at a pirate point in the system. The new vessel was inside the picket, and successfully deployed DropShips onto the northern continent of Cranston, from which emerged the 3rd Canopian Light Horse, fresh and supposedly the garrison for Restitution according to SLDF intelligence. Suddenly faced with twice the number of enemy forces and completely outmatched in the air thanks to the significant AeroSpace forces accompanying the 3rd Light Horse, the 58th Brigade commander decided to split his forces; General Michael Martinnson deployed two thirds of his forces against the 1st Canopian Grenadiers, while ordering the other third and both Marik regiments to prevent the 3rd Canopian Light Horse from linking up with the Canopian Grenadiers. The next three weeks were a brutal campaign, particularly for the scratch company of AeroSpace pilots and fighters that were all the 58th and the 2nd Orloff Grenadiers could muster to defend them from the Canopian AeroSpace assets; by the time the MAF defenders had been forced to retreat from Cranston, three weeks after the 3rd Light Horse's arrival, the 58th had been bloodied, the MAF had proved that the war was going to be fought in a fashion entirely different to that predicted by the SLDF planners, and the surviving SLDF AeroSpace pilots from the scratch company were the first recipients of the Star League Medal of Honor in the Canopian theatre.[3][4]

In contrast with the difficult campaign to capture Cranston, the 58th Brigade deployment to capture Ruschegg in 2578 was relatively easy; one of three former Free Worlds League which had defected to the Magistracy in the 2550s, all three were conquered through a display of overwhelming force and agile diplomacy by Captain-General Marik. The defenders of Ruschegg effectively offered no resistance to the SLDF landings, although it would have been difficult for any local militia to contest an invasion which saw the 58th deploy alongside the 62nd Royal Brigade and no less than four regiments of FWLM regulars, including two regiments from the Marik Militia and two brigades from the Fusiliers of Oriente. Within a very short period of time the planetary leader of Ruschegg had not only surrendered but was actively petitioning for Ruschegg to be admitted into the Free Worlds League.[1][5]

The MAF had managed to surprise and outfox the SLDF task force several times in the opening years of the war, and as a result the SLDF timetable slowed as Captain-General Marik built up her forces for the next major invasions. In addition to refitting and rearming VII Corps and the FWLM regiments acting in support, Marik targeted two worlds each year for invasion, consolidating gains as part of a slow encirclement of the Magistracy capital world of Canopus IV. The 58th was involved in two of these four campaigns; in September 2580, the 58th landed on Aspropirgos alongside the 3rd Marik Militia and the 2nd Brigade of the Fusiliers of Oriente, clashing with the 1st Canopian Fusiliers and the 2nd Canopian Grenadiers. In what would be the standard pattern of warfare for the MAF when defending worlds during 2580-2581, the two Canopian regiments did their best to sting the 58th and the supporting Marik regiments before retreating ahead of any effective retaliation, saving their strength for later battles. The same thing happened when the 58th landed on Borgan's Rift in September-October 2581, accompanied again by the 3rd Marik Militia and with the 402nd Independent Regiment also providing support; this time the 58th faced two of the mercenary units employed by the MAF, The Red Hand and Kincani's Dragoons, but the campaign was a repeat of the battle for Aspropirgos.[1][4][6]

Tetski was an important world to the Magistracy and to the MAF in particular; Tetski was not just an industrialized world, but more importantly it was the site of the largest BattleMech repair and refurbishment facilities outside of the Canopus IV. The MAF regularly moved regiments and battalions through Tetski for repair and refit, and capturing Tetski would deal a heavy blow to the MAF. As a result, Tetski was one of the few worlds to have a permanent naval picket in orbit in addition to any defending ground forces. The SLDF campaign to capture Tetski began with the insertion of two battalions of the 1st Marik Militia onto Tetski in early September 2582, a distraction intended to tie up the defending 3rd Canopian Light Horse. After five days of extended combat across the wilds of Tetski, the SLDF launched the second stage of the invasion, launching a surprise assault where a large WarShip contingent tore through the defending picket, destroying both vessels before dropping the 58th Brigade onto the planet along with two more regiments of FWLM troops. The 3rd Light Horse made a high-speed run to the capital city of Tokhai and the vital BattleMech repair facilities there, arriving just minutes ahead of the 58th. Refusing calls to surrender in the face of overwhelming numbers, the 3rd Light Horse fought to the last man, inflicting heavy losses on the 58th and the Marik regiments before being destroyed.[1][4][7]

The battle for Canopus IV was the last major campaign for the 58th Brigade during the war with the Magistracy. In battle from the first day of combat, the 58th was responsible for the destruction of the 1st Canopian Grenadiers, the unit that the 58th had battled several years before on Cranston. The 58th pushed the 1st Grenadiers back against the Thetis River and then systematically reduced the Canopian regiment to a fraction of it's former self, with just two companies of Canopian 'Mechs surviving to retreat across the river. After the defeat and surrender of the Magestrix and the defending Canopian forces on Canopus, the 58th wouldn't fight another significant battle before the end of the war.[2][8]

Despite the doctrine and organization originally defined during the creation of the SLDF, most SLDF brigades were a mélange of combined arms units.[9] However, the 58th appears to have been formed entirely from BattleMech regiments.[3]

The 58th is described during the invasion of Cranston as being composed of 9 BattleMech regiments.[3] This is three times the number of regiments normally seen in an SLDF brigade,[9] and has been queried at the Catalyst Game Labs forum.